The present invention generally relates to SiAlON-based ceramics useful as cutting tools for the machining of metallic materials.
Cutting tools with high wear resistance and reliability are critical to improving industrial productivity. It has been found that ceramic cutting tools allow considerable increase in the rate of machining or improvements in the dimensional tolerances achieved through reduction in wear of the tool.
Such ceramic cutting tools are made from alumina, alumina-titanium carbide composites, silicon nitride or SiAlON. Of these, the alumina and alumina titanium carbide composites exhibit very good wear performance due to their high hardness but suffer from very poor reliability due to their tendency to chip. The SiAlON and silicon nitride grades are considered more reliable because they show less tendency to chip.
However, existing ceramic cutting tools are inadequate due to their poor combination of hardness and toughness and processability. There remains a need for a ceramic material with a combination of high wear resistance and reliability provided by a combination of high hardness and resistance to chipping.
a) Desired Properties of Ceramic Cutting Tools
It is taught, (see for example K. Jack, xe2x80x9cCeramic Cutting Tool Materialsxe2x80x9d, Materials and Design Vol. 7, September/October 1986, pg. 267-270, see especially pg. 270. and C. Chatfield, T. Ekstrom and M. Mikus, J. Mat. Sci. 21, pg. 2297-2307 (1986)) that the properties of interest in metal cutting inserts are resistance to mechanical and thermal shock, resistance to plastic deformation at high temperatures, on the order of 1000xc2x0 C., resistance to abrasive wear, and resistance to chemical or dissolution wear. Resistance to mechanical shock is improved by high toughness, while resistance to abrasive wear is improved by both high toughness and high hardness. Lack of toughness leads to inserts being unreliable because they are susceptible to chipping, while too low a hot hardness can result in failure due to excessive plastic deformation. Low hardness results in poor resistance to abrasive wear as discussed below. Resistance of oxide based cutting tools such as alumina or alumina titanium carbide composites to chemical wear while machining steel is much better than that of silicon nitride or SiAlON.
The wear of a ceramic is taught by S. T. Buljan and V. K. Sarin (xe2x80x9cDesign and Wear Resistance of Silicon Nitride-Based Compositesxe2x80x9d, Inst. Phys. Conf. Ser. 75, Chap. 9, pg. 873 (1986)) to be related to the hardness and toughness of the material according to a factor Kic3/4 H1/2 where H is hardness and Kic is fracture toughness, with improved wear resulting fro higher hardness and higher toughness.
Ceramic materials made from SiAlONs typically have mechanical and physical properties similar to those of beta silicon nitride, including a low thermal expansion, but chemical properties similar to aluminum oxide (see for example, K. Jack, xe2x80x9cSialons and Related Nitrogen Ceramicsxe2x80x9d, J. Mat. Sci. 11 (1976) 1135-1158, pg. 1146).
b) SiAlON: Silicon Nitride with Alumina
xe2x80x9cSiAlONxe2x80x9ds are phases in the silicon-aluminum-oxygen-nitrogen and related systems. SiAlON ceramic materials differ from silicon nitride because aluminum and oxygen are contained in the crystal structure (see K. Jack, xe2x80x9cSialons and Related Nitrogen Ceramicsxe2x80x9d J. Mat. Sci. 11 (1976) 1135-1158, and F. Riley, xe2x80x9cSilicon Nitride and Related Materialsxe2x80x9d, J. Amer. Ceram. Soc. 83 [2] 245-65, February 2000).
Ceramic items made from SiAlON typically have high elevated temperature strength and hardness and are desirable for industrial applications. In particular SiAlON retains hardness at high temperatures better than alumina (see for example Jack, Materials and Design, Vol 7, No 5, October 86, pg. 271, FIG. 10).
In addition to the aluminum and oxygen incorporated into the structure, compounds such as yttria are commonly added to assist sintering. During sintering these compounds react with silica on the surface of the silicon nitride, silica deliberately added or otherwise present as an impurity. Similarly such compounds react with any alumina present, either deliberately added or present on the surface of aluminum nitride, or otherwise added as an impurity.
These additional elements greatly increase the complexity of the phase relations affecting SiAlON materials and thus increase the difficulty in processing SiAlON materials to achieve the desired properties. It is known, for example, that the phase chemistry of the intergranular phases in SiAlON is more complex than that of the corresponding silicon nitride ceramic systems (see for example F. Riley J. Amer. Ceram. Soc. 83 [2] pg. 259, February 2000). On the other hand, the complexity of the phase relations for these materials enables articles made from SiAlONs to be fabricated with much more economical processes. For example, in the case of silicon nitride, dense bodies can generally only be made by hot pressing or the use of high gas pressure sintering techniques to prevent the decomposition of the silicon nitride phase during densification. The SiAlON material typically may be processed to a high density without the application of high pressures. This process is typically known as pressureless sintering and consists of cold pressing followed by sintering at normal atmospheric pressures of an inert gas. The use of this process enables considerable reduction in the cost of fabricated articles.
The complex phase relations of the SiAlON materials makes it very difficult to accurately or definitively define the nature of the crystal structure in a finished ceramic. Thus it is useful and common to define such ceramic compositions in terms of the raw materials from which they are fabricated (i.e., formulations) in addition to attempting to fully characterize the finished materials.
c) Alphaxe2x80x2 and Betaxe2x80x2 Phases of SiAlON
The two best known crystal phases in the SiAlON family are the alphaxe2x80x2 and betaxe2x80x2 phases, based on corresponding alpha and beta silicon nitride crystal structures. In these SiAlON phases a portion of the silicon and nitrogen atoms are replaced by aluminum and oxygen atoms.
The betaxe2x80x2 SiAlON phase is generally considered to be represented by the formula Si6xe2x88x92zAlzOzN8xe2x88x92z, wherein 0 less than z less than 4.2. This structure does not incorporate additional metal ions in the crystal lattice.
Microstructurally, betaxe2x80x2 SiAlON mostly appears as elongated high aspect ratio fiber like grains which contribute to high strength and toughness in the ceramic material.
Ceramic articles made from betaxe2x80x2 SiAlON can show high values of toughness but show low hardness, that is their hardness is, for example, on the order of 92 Rockwell (A scale) (see U.S. Pat. No. 4,547,470 to Tanase et al.). As a result of the low hardness such ceramic cutting tools do not show satisfactory wear resistance.
The alphaxe2x80x2 SiAlON phase is generally considered to be represented by the formula Mx(Si,Al)12(O,N)16 wherein 0 less than x less than 2 and M is an element such as Mg, Y, Ce, Sc or other rare earth elements. More precisely, the crystal stoichiometry is represented by Mm/vS12xe2x88x92mxe2x88x92nAlm+nOnN16xe2x88x92n (see G. Z. Cao and R. Metselaar, xe2x80x9cAlphaxe2x80x2-Sialon Ceramics: A Reviewxe2x80x9d, Chem. Mat. Vol 3 No 2, 242-252 (1991)), wherein v is the valence of M. The two formulas are used interchangeably in this specification. This structure accommodates additional M ions that are not accommodated within the betaxe2x80x2 SiAlON structure.
Typically alphaxe2x80x2 SiAlON appears mostly as equiaxed grains in the microstructure of the ceramic and is associated with higher hardness in the material. This equiaxed microstructure does not provide the high toughness associated with the fiber-like betaxe2x80x2 SiAlON microstructure.
Thus, in attempts to provide ceramic SiAlON compositions which are usable in high temperature applications such as cutting tools, various authors and patentees have taught the combination of alphaxe2x80x2 SiAlON with betaxe2x80x2 SiAlON. Thus, it is taught by U.S. Pat. No. 5,413,972 to Hwang et al., (at col. 1, 1. 39 to 43) and F. Riley, (xe2x80x9cSilicon Nitride and Related Materialsxe2x80x9d, J. Amer. Ceram. Soc. 83 [2] 245-65, February 2000), that by varying the starting materials, it is possible to vary the alphaxe2x80x2 and betaxe2x80x2 phases and hence the hardness and strength can be tailored.
It is taught by Ekstrom et al. (xe2x80x9cMixed alphaxe2x80x2 and betaxe2x80x2 (Sixe2x80x94Alxe2x80x94Oxe2x80x94N) Materials with Yttria and Neodynia Additionsxe2x80x9d, Mat. Sci. and Eng. A105/106 (1988) 161-168), that mixed alphaxe2x80x2 and betaxe2x80x2 SiAlON materials exhibit benefits compared with pure betaxe2x80x2 SiAlON ceramics in engineering applications such as cutting tools. This is attributed to the higher hardness associated with the presence of the alphaxe2x80x2 SiAlON phase (see Chatfield, Ekstrom and Mikus, J. Mat. Sci. 21, pg. 2297-2307 (1986)). In order for ceramic materials such as these to achieve useful properties, the ceramic must be made to near theoretical density which in turn requires sintering aids. It is also taught that the most successful sintering aids used are yttria or yttria plus alumina. The compositions which allow pressureless sintering of fully dense ceramic materials consisting of alphaxe2x80x2 and betaxe2x80x2 SiAlON phases with yttria sintering aids are well established. For example, U.S. Pat. No. 4,327,187 to Komatsu et al. teaches the use of yttria, alumina and AlN in silicon nitride ceramic formulations, and producing a sintered ceramic body having greater than about 95% of theoretical density by adding quantities of TiO2, MgO or ZrO2. This patent states that a density of  greater than 95% can be obtained with good retention of hot strength by an undefined pressureless sintering method; however, a method to achieve a useful product simultaneously having high density, high toughness and high hardness is not disclosed.
U.S. Pat. No. 4,711,644 and U.S. Pat. No. 4,563,433, both to Yeckley et al., teach that yttrium is the most desired additive to make a dual phase alphaxe2x80x2 and betaxe2x80x2 SiAlON material and cutting tool because it xe2x80x9cproduces high melting glasses with the silica and alumina present and allow the material to be used at higher temperature than would be possible with low melting glassxe2x80x9d (see U.S. Pat. No. 4,563,433, col. 4, 1. 9).
d) Toughness
It is known that the high temperature properties and the room temperature fracture toughness of silicon nitride and related ceramics depends on not only the ratio of the alpha to beta phase and the size and shape of the beta phase grains but it also depends on the amount and nature of the minor phases disposed between the grains of the alpha and beta phases (see Kleebe et al., J. Amer. Ceram. Soc. 82 [7] 1857 (1999)).
e) Degradation of Properties
Many papers and patents note that a common problem is that the intergranular phases degrade the properties of ceramics. For example, U.S. Pat. No. 5,413,972 to Hwang et al; D. Dressler and R Riedel, Int. J. Refractory Metals and Hard Materials 15 (1997), pg. 13-47 especially pg. 23; and D. A. Bonnel et al., J. Amer. Ceram. Soc. 70 (1987), pg. 460, all teach that these intergranular phases are undesirable because they generally cause high temperature degradation and reduction in strength.
Riley (J. Amer. Ceram. Soc., February 2000, pg. 259) notes that a distinguishing feature of the mixed alphaxe2x80x2 and betaxe2x80x2 SiAlON system is that the conversion of the alphaxe2x80x2 SiAlON to betaxe2x80x2 SiAlON releases glass because the alphaxe2x80x2 phase can accommodate other metal oxides while the betaxe2x80x2 phase cannot. As a result, the high temperature properties are expected to deteriorate. For example, U.S. Pat. No. 4,818,635 to Ekstrom et al., teaches SiAlON materials for cutting tools that can be sintered without pressure by adding alumina and small additions of metal oxides, nitrides, oxynitrides of Y, Ca, Mg, Be, lanthanides etc., or mixtures thereof. This reference also teaches that the amount of glass must be small, but not so small as to affect the toughness behavior. Certain metals are taught to lower the softening of the glass phase, for example Ca, Mg, Fe etc. In order to obtain a glass phase having optimum high temperature properties, the content of such elements must be small. As stated therein: xe2x80x9cAdditions of Mg compounds will, for instance give a ceramic material which is more easily processed . . . . However the material will lack good high-temperature properties.xe2x80x9d (see col. 2, 1. 51-55).
f) Prior Art Attempts to Avoid Degradation of Properties
Prior attempts to overcome the above limitations are as follows:
Eliminating or minimizing these intergranular materialsxe2x80x94For example, patents such as U.S. Pat. No. 5,413,972 to Hwang et al., teaches eliminating or minimizing intergranular phases by controlling the starting materials. However these methods produce ceramic bodies that are difficult if not impossible to fully densify. It is also taught that eliminating the additives changes the microstructure and impairs the mechanical properties (see col. 2, 1. 1-5).
U.S. Pat. No. 4,563,433 to Yeckley et al., teaches complicated methods of sintering and materials containing certain defined xe2x80x9cglassy phase,xe2x80x9d with a minimum hardness of 92.5. However the process is very difficult to apply in manufacturing, and the hardness is insufficient for practical application for cutting tools.
Crystallizing these intergranular materialsxe2x80x94However it is taught, (see for example Chatfield et al. supra pg. 2302) that reduction in properties is associated with crystallization. The article states: xe2x80x9cPost heat treatment above 1400 K causes the glass to partially re-crystallize into YAG. The toughness of the material decreases and the cutting tool performance in turning cast iron drops drastically.xe2x80x9d
Adding larger amounts of AlN (see T. Ekstrom and M. Nygen, xe2x80x9cSiAlON Ceramicsxe2x80x9d 75 [2] J. Amer. Ceram. Soc. 259 (1992))xe2x80x94These methods suffer from the problem that complete crystallization may be inhibited by kinetic factors and do not reduce the glass content sufficiently to be effective. Such methods are further complicated in SiAlON materials because of their complex phase relations which in turn can produce numerous undesirable phases with even slight changes in starting compositions (see U.S. Pat. No. 5,413,972 to Hwang et al., col. 2, 1. 44-48).
Providing a dispersed phase to restrict or modify the grain sizexe2x80x94Thus, for example, U.S. Pat. No. 4,547,470 to Tanase et al., discloses ceramic SiAlON-based materials having a dispersed phase selected from the carbides of Ti, Zr or Hf, nitrides thereof, carbo-nitrides thereof, or carbo-oxy-nitrides thereof, where the dispersed phase is intended to restrain the growth of the SiAlON phases. This approach restricts the growth of the fiber-like betaxe2x80x2 grains and thus will reduce the toughness of the ceramic.
Providing a very refractory intergranular phase by the use of specific rare earth (RE) oxides and/or the formation of specific secondary phasesxe2x80x94For example in U.S. Pat. No. 5,200,374, to Kohtoku et al., discloses a SiAlON based sintered body having a high mechanical strength and fracture toughness comprising a first phase of RE-alphaxe2x80x2 SiAlON, a second phase of betaxe2x80x2 SiAlON and a third crystal phase containing at least one rare earth element (RE), wherein RE is Ho, Er, Tm, Yb or Lu. The third phase is RE2M2-UO7-2U, where M is at least one of Hf, Zr and U. This approach also has the limitations that it is difficult to achieve due to the complexity of the phase system and difficult if not impossible to process into a useful article.
The above approaches typically produce ceramic bodies that have inferior properties or are difficult if not impossible to fully densify and fabricate into useful products.
In summary, with respect to the use of MgO, U.S. Pat. No. 4,327,187 to Komatsu et al., lists MgO as an aid for the densification of silicon nitride-based ceramics. Alphaxe2x80x2 plus betaxe2x80x2 SiAlON composite ceramics are taught in U.S. Pat. No. 4,711,644 and U.S. Pat. No. 4,563,433 to Yeckley et al., but MgO is contraindicated as causing degraded high temperature properties. MgO is also contraindicated by U.S. Pat. No. 4,818,635 to Ekstrom et al. Multi-cationic mixtures in the alphaxe2x80x2 SiAlON crystal structure are known from U.S. Pat. No. 5,413,972 to Hwang et al., but the presence of a third non-alphaxe2x80x2 SiAlON, non-betaxe2x80x2 SiAlON intergranular phase is taught away from as resulting in degraded properties. As well, Huang et al. teach the requirement of pressure sintering in order to achieve a dense ceramic.
A new ceramic material has been discovered by the inventors having a surprisingly excellent combination of high hardness, significantly enhanced toughness and utility for the high speed machining of metals. The new material is a SiAlON ceramic material having a SiAlON matrix comprising:
a) a phase of alphaxe2x80x2 SiAlON represented by the general formula of Mx(Si,Al)12(O,N)16, wherein 0 less than x less than 2 and M is at least two cationic elements, a first cationic element being Mg and optionally one or more of Ca, Sr, and Ba, and a second cationic element being one or more of Y, Sc, La and the rare earth (RE) elements;
b) a phase of betaxe2x80x2 SiAlON represented by the general formula Si6xe2x88x92zAlzOzN8xe2x88x92z wherein 0 less than z less than 4.2; and
c) a component containing glass, and at least one additional intergranular crystal phase that is detectable using X-ray diffraction (XRD) techniques,
wherein the amount of the first cationic element is 0.2 to 4 weight percent (more preferably 0.3 to 3 weight percent, most preferably 0.4 to 2.5, calculated as an element and based on the SiAlON matrix, and the amount of the second cationic element is 0.5 to 15 weight percent, calculated as an oxide (more preferably 3 to 10 weight percent, most preferably 4 to 8 weight percent), based on the SiAlON matrix.
The above amounts for M as used herein and in the claims are meant to refer to amount as included in the xe2x80x9cas formulatedxe2x80x9d composition, based on the SiAlON matrix phase, that is based on the combined alphaxe2x80x2 and betaxe2x80x2 SiAlON phases and the component c).
The term xe2x80x9crare earth (RE)xe2x80x9d as used herein and in the claims means the rare earth elements having atomic numbers between 57 and 71, but excluding Ce.
Preferably the first cationic element is Mg alone. Preferably the second cationic element is one or more of Sc, Y, La, Yb, Sm, Nd, and Pr, more preferably Y or Yb, and most preferably Y. It is discovered that the Mg is distributed between the alphaxe2x80x2 SiAlON phase and the component c). This, as well as the detection of the intergranular crystal phase, can be confirmed by transmission electron microscopy (TEM).
A significant and surprising advantage of the present invention is the unexpected results of using Mg to form one or more intergranular crystal phases that may be detected by XRD. Such elements were previously considered by the prior art to be harmful to the properties of the SiAlON body by the formation of low melting glasses, as described above.
It should be understood that no assertion is being made that any metal, oxide or nitride exists as separate phases within the ceramic unless explicitly described as a separate or dispersed phase. Thus, a reference to an amount of a component expressed as a metal, oxide or a nitride is made for the purposes of calculation only, without implying that the component is present in that form in either a precursor or final formulation.
Depending on the application for the SiAlON ceramic material of the invention, the ceramic material of this invention may also contain a substantially inert filler such as a known oxide, nitride, silicide, carbide, carbo-oxy-nitride, oxy-carbide, carbo-nitride, or boride of one or more of the elements Ti, Zr, Hf, Nb, Ta, V, Cr, Mo, W, B, Si. By xe2x80x9cknownxe2x80x9d is meant only those compounds which are known to exist, thus excluding impossible or improbable combinations such as borides of boron. Preferably the inert filler is one or more of TiN, Ti(C,N) (with the atomic ratio of C:N between 0 and 1) Mo2C, TiC and SiC, with TiN, Mo2C and Ti(C,N) being most preferred. The inert filler is included in amounts from 1 to 50 volume percent, based on the final ceramic material. Preferably such additional particles may constitute between 1.5 and 40 volume percent. Most preferably the range is between 2 and 25 volume percent. The inclusion of the inert filler may result in somewhat softer SiAlON ceramic materials for use as a composite which contains the SiAlON matrix phases with the filler in a dispersed phase.
It has been found that the new SiAlON ceramic material can provide wear performance better than that of previously known SiAlON and/or silicon nitride cutting tools. A significant and unexpected advantage of this new material is that it combines high wear resistance and fracture resistance with low cost since it may be easily fabricated by the inexpensive cold pressing and sintering method.
Ceramic materials of the present invention having the best wear and fracture resistance properties are formed by microwave sintering, which avoids the necessity of pressure sintering.
The invention also extends to a method of preparing a SiAlON ceramic material comprising:
a) providing a powder mixture of:
i. silicon nitride as the major ingredient;
ii. 0.1 to 20 parts by weight of an oxide or nitride of Sc, Y, La or a RE;
iii. 0.1 to 20 parts by weight of aluminum nitride;
iv. 0.1 to 6.5 parts by weight of an oxide or nitride of Mg and optionally of one or more of Ca, Sr, and Ba;
b) forming a green compact from the powder mixture; and
c) heating the green compact to form a ceramic material with closed porosity.
The heating step preferably comprises:
c1) an optional first heating step at about 300 to 900xc2x0 C., preferably about 600xc2x0 C., in a static or flowing non-reactive atmosphere;
c2) a second heating step at between 1500 and 1800xc2x0 C. in a static or flowing non-reactive atmosphere; and
c3) an optional third heating step in a hot isostatic press at a temperature between 1400 and 2000xc2x0 C. under a pressure of a non-reactive gas at a pressure of between 690 KPa and 413 MPa.
Heating is preferably accomplished by microwave sintering in a flowing non-reactive gas at a temperature of 1650-1800xc2x0 C., in order to provide the best properties and to avoid the need for pressure sintering.
It is also a surprising discovery of this invention that the use of alumina, or compounds containing alumina such as magnesium aluminate spinel, in the formulation does not produce the desired combination of properties. Thus, the formulation is most preferably substantially free of alumina or compounds thereof. While aluminum nitride is a preferred material for formulations of the present invention, other ingredients can be considered substantially free of alumina if the equivalent aluminum oxide content is less than about 1.5, more preferably less than 1, weight percent of the finished ceramic material.
The invention also extends to cutting tools and cutting tool inserts prepared from the ceramic materials.
In accordance with this invention a ceramic metal cutting insert is provided for chip forming machining of metallic materials. The material has a hardness of greater than 92 Ra, and for some applications greater than 93.5 Ra, or greater than 94 Ra. The ceramic material also has an indentation fracture toughness of greater than 6.5 MPam1/2, preferably greater than 7.0 MPam1/2 and most preferably greater than 7.5 MPam1/2. The ceramic material preferably has a density greater than 98% theoretical, and more preferably greater than 99% theoretical.